With the increasing interest in high-speed vehicles for inter-urban, intra-urban and rural-urban transport of passengers and freight, considerable attention has been directed to avoiding frictional drive systems between the vehicle and a track to reduce the friction forces between them.
To this end, as described in the earlier applications mentioned above, such vehicles have been suspended by magnetic forces from the track which can be provided on opposite sides with a pair of substantially continuous armature rails. The latter are juxtaposed with rows of suspension electromagnets on each side of the vehicle and a substantially constant suspension gap is maintained between the cores of the electromagnets and the armature rails by suitable circuitry. For lateral guidance of the vehicle on the track, the suspension electromagnets and armature rails may be shaped as described in the aforementioned copending applications or additional laterally effective guide electromagnets and corresponding armature rails may be provided.
Such suspension and guide systems avoid direct contact of the vehicle and the track except for wipers or the like which may be provided to enable the vehicle to pick up electric current from the track.
Similarly it has been attempted to replace the rotary drive motors of conventional vehicles with linear-induction motors designed to apply a propelling force to the vehicle without moving parts other than the stator carried by the vehicle body and a reaction rail provided on the track. While the present invention is concerned primarily with linear-induction motors in magnetic-suspension or magnetic-levitation systems, it should be noted that the principles here disclosed may be equally applicable to other vehicle-drive arrangements.
Linear-induction motors of the type described operate in accordance with eddy-current principles whereby the magnetic field bridging the stator and the reaction rail induces an eddy-current in a conductive layer of the rail or in the entire rail. This eddy-current reacts with the magnetic field and, by causing the field to move along the stator, i.e. by the use of a plurality of coils energized in a rotary-field multiphase system, a linear force is produced between the stator and the rail which, since the rail is fixed, propels the vehicle along the track.
In earlier linear-induction motor constructions, the reaction rail has been of U figuration with a channel open to one side to receive the stator and a plurality of annular windings spaced therealong, the windings or coils lying in places perpendicular to the direction of displacement and to the U section rails. The coils were mounted in grooves of a core structure of ferromagnetic material so that the upper and lower flanks of the stator, active surfaces are formed which interact with rails provided along the inner flanks of the shanks of the U. These layers are conductive to generate the aforementioned eddy-currents.
As noted in application Ser. No. 425,615, the use of such a linear-induction motor in high-speed magnetic-suspension vehicles is not practical because the mechanical characteristics of the motor prevented it from being used at the desired high speeds, nor can the motors be constructed to have a capacity in in the magawatt range.
The improvement described in application Ser. No. 425,615 overcame these disadvantages by providing a linear-induction motor, especially for a magnetic-levitation vehicle, which comprises a U-section channel or rail coated or clad along three of its internal surfaces with a reactive layer of electrically conductive material and receiving the stator of the motor which was cantilevered from the vehicle. The stator comprised a plurality of axially spaced annular coils (ring windings) energized by a multiphase source. The active magnetic mass of this stator included a core surrounded by the coils and extending into the channel or rail while being subdivided along a longitudinal plane into two core-sheet stacks with a lamellae of the two stacks being oriented in different mutually orthogonal directions. The two stacks or packets were jointed together along the longitudinal place into a rigid core structure.
An earlier linear-induction motor had stator windings lying only in places at the back of the stator core or lamination stacks and was characterized by high leakage inductance and low power. Increase in the stator current to increase the power led to comparatively high air-gap induction losses.
A somewhat more effective linear-induction motor was described in U.S. Pat. No. 3,333,124 which has a number of annular windings which are arranged in succession in the longitudinal direction of the motor and between each two such windings there is provided a rectangular stator lamination stack.
The stator lamination stacks are attached to an assembly plate extending in the longitudinal direction of the motor.
In such prior-art constructions the pole surfaces of the stator in comparison with the pole surfaces of the reaction rail are relatively small so that the magnetic-flux densities in the stator poles are relatively high, especially with high electrical power, thereby increasing the air-gap induction and resulting in power losses.
Accordingly, the prior art linear-induction motors are of relatively lower efficiency.
In addition, the mechanical structure of prior art linear-induction motors has left much to be desired, especially where the individual stator lamination stacks are attached to an assembly plate by three comparatively small end surfaces so that the center of gravity of the stator lamination stacks is spaced comparatively far from the point of support. Such systems are not suitable for the propulsion of high-speed vehicles and especially high-speed magnetic-suspension vehicles.